Within the context of the invention, heat treatment should be understood to mean any technique for the treatment of a substrate material which involves cooling at least part of the said substrate material, especially: surface coating, tempering, nitriding, carburizing, plasma spraying, oxycutting, laser cutting, HVOF (high-velocity oxyfuel) spraying, flame spraying, etc. These various heat-treatment techniques are known and widely used in the industrial field.
For example, in order to produce coatings on materials, a spray jet, consisting of hot carrier gas and of molten or softened particles of the coating material, is directed onto the surface of the material or substrate material to be treated, which surface is cooled before and/or after treatment by a jet of a coolant, such as carbon dioxide (CO.sub.2) or liquid argon (as shown in FIG. 5).
Thus, plasma spraying is widely used to produce coatings on any type of material, such as composites, for example resins or plastics, which have to be coated with thin ceramic layers or metal layers.
This technique is also used for producing shielding coatings in the mechanical engineering field, for example in the aeronautics or automobile industries, or in the energy industry.
The technique of thermal spraying involves very high temperatures and high heating powers. This is because the spray jet, generally composed of a hot carrier gas and of particles of the coating material, must be at a temperature high enough to make it possible to soften or melt the particles of added coating material and, moreover, to achieve effective heat treatment of the surface of the material or of the component that is to be coated.
The material that is to be coated, or substrate material, therefore heats up considerably because, on the one hand, of the amount of heat provided directly by the hot gases and because, on the other hand, of the at least partially molten coating particles which, when they come into contact with the substrate material, transfer a large amount of heat to the latter in a very short space of time. Conventionally, the substrate material heats up several hundreds of degrees and a thermal equilibrium is established, on the one hand, by heat exchange with the ambient atmosphere and, on the other hand, by diffusion of the heat through the substrate material and through the coating layer.
It has been observed that the adhesion, that is to say the bonding of the coating layer to the substrate material, increases with temperature. In other words, the higher the temperature, the more the mechanical anchoring and the chemical reactions, which are established between the coating layer and the substrate material, change in a favourable way and, consequently, result in more effective rapid cooling.
This may be explained in particular by better wettability of the molten coating particles and therefore better spreading or flattening of the latter on the substrate material.
However, it is also necessary to consider the microscopic properties not only of the coating layer but also of the substrate material.
This is because, when successively stacking several coating layers or laminations, internal tensile stresses are generated in the coating, these stresses being higher the lower the thermal conductivity of the coating material deposited or the thicker the material deposited at each pass.
Such a phenomenon has, in particular, been observed when ceramic-type particles are sprayed, where the appearance of cracks or even, in some cases, delamination of the coating, due to too high a temperature difference between, on the one hand, the substrate material and the coating layer and, on the other hand, in the actual ceramic coating layer, have been observed.
Likewise, during cooling, if the difference in thermal expansion coefficient (.alpha.) between the substrate material and the coating material becomes too great, residual stresses are generated at the interface between the substrate material and the coating material, which stresses are the cause of disbandment and delamination phenomena.
This has been observed, in particular, when spraying a material of the ceramic type onto metals, such as aluminum, where it has been observed that the ceramic layer (.alpha.=8.times.10.sup.-6 K.sup.-1) cannot adhere to a component made of aluminum (.alpha.=22.times.10.sup.-6 K.sup.-1) if the temperature of the substrate exceeds a few hundred degrees.
From this it follows that the thickness of the coating cannot, in some cases, exceed a few tenths of a millimeter, thereby greatly limiting the possible industrial applications.
This is because, when the coating layer is intended to act as a thermal barrier, i.e. as a thermal insulator, it must, in some cases, have a thickness well above one millimeter, something which is, consequently, not achievable.
It should therefore be understood that the properties of the substrate material to be coated should also be taken into consideration, in particular the thermal expansion coefficient and the thermal conductivity, the latter reflecting the ability of the material to remove heat.
In order to help to solve the aforementioned problems, it is common practice to combine the operation of heat-treating the material, such as the deposition of a coating layer, with an operation of cooling the substrate material before and/or after treating the latter, i.e., for example, spraying the jet comprising the hot carrier gas and the at least partially molten particles of coating material onto the surface of the substrate material.
The use of additional cooling makes it possible, furthermore, to apply the thermal spraying technique to the coating of so-called "sensitive" substrate materials, on which the temperature may have an undesirable effect, such as organic materials or composites, paper or wood, or low-melting-point metals, such as aluminum or copper.
In other words, one of the aims of the additional cooling is to make it possible, by quenching, for heat to be removed effectively and, in all cases, more rapidly than by leaving the component to be treated to cool down by itself, away from the jet of hot gas.
Furthermore, additional cooling makes it possible to establish a much lower temperature gradient between the coating layer and the substrate material, thereby greatly improving the integrity of the coating.
Moreover, using effective cooling also enables the spraying time, and therefore the costs, to be considerably reduced, given that it is no longer necessary to wait until the treated components cool down by themselves; it has been possible, in some cases, to reduce the spraying time by a factor of 10.
Currently, there are several cooling techniques using coolants of different types, according to the effectiveness of the desired cooling, which effectiveness, as we saw above, depends on the intrinsic properties of the substrate material/coating layer pair.
However, care should be taken, in all cases, to ensure that the jet of cooling air disturbs the hot gas as little as possible, i.e. the mixture consisting of one or more hot gases and, in general, molten or softened particles, so as to avoid cooling the jet, oxidizing the molten coating particles, contaminating the coating layer, etc.
However, as we saw previously, the temperature of the substrate material is a critical parameter given that, if its temperature exceeds a certain value, the substrate material may undergo irreversible degradation.
Thus, when it is desired to produce less intensive cooling, that is to say the component to be treated being left at a temperature lying within the range from 150.degree. C. to about 600.degree. C., which is, moreover, easy to implement and inexpensive, it is possible to use compressed air as the coolant.
On the other hand, compressed air is not suitable if it is desired to have good cooling effectiveness.
One alternative therefore consists in using a coolant such as argon or carbon dioxide (CO.sub.2) as the coolant.
This is because liquid argon used as the coolant makes it possible to maintain the temperature of the substrate material and/or of the coating layer at a temperature generally between 0 and 150.degree. C., this temperature essentially depending on the pressure of liquid argon and the flow rate of gaseous argon employed, which conditions ensure that the stream of liquid argon is atomized into fine droplets of variable diameter. Such cooling effectiveness allows a very thick coating layer to be deposited, for example a layer about 3 mm thick.
However, the use of liquid argon as the coolant increases the production costs and requires more expensive equipment. Thus, the use on an industrial scale of argon, as the coolant, is generally limited to the heat treatment of components with a high added value.
Apart from argon, it is also possible to use carbon dioxide as the coolant.
The use of CO.sub.2 is highly advantageous since, on the one hand, it performs in a similar way to argon, given that the temperature of the substrate material may be maintained at values of about the ambient temperature and, on the other hand, it costs markedly less than argon. Such CO.sub.2 cooling can therefore be applied to the heat treatment of all kinds of components, whatever their added value. By way of example, mention may be made of depositing a silica, titanium dioxide or molybdenum coating layer, with a thickness of 1 to 1.5 mm, on a steel component, or else depositing an approximately 3 mm layer of zirconia, which is to act as a thermal barrier, on a component made of an aluminum alloy.
Furthermore, CO.sub.2 cooling is also well suited to depositing thin coatings on substrate materials having a high expansion coefficient, such as aluminum alloys. By quenching in a cryogenic liquid, it is then possible to separate the coating from the substrate material.
Likewise, when using an HVOF-spraying heat-treatment process, CO.sub.2 cooling allows, in particular, deposition of a tungsten/cobalt carbide coating layer on a substrate material while preventing the formation of a carbide prejudicial to the desired properties, namely, in particular, the wear resistance.
This CO.sub.2 cooling also allows deposition of a chromium/nickel layer on aluminum components, something which cannot be carried out using compressed air, given that the difference in expansion coefficient between the chromium/nickel coating layer and the aluminum component demands keeping the temperature below 80.degree. C.
Furthermore, the use of CO.sub.2 cooling also prevents copper from becoming highly oxidized when the latter is used as a coating material for producing a thick coating layer, i.e. about 2 mm.
In some cases, it is also possible to use liquid nitrogen as the coolant.
Many thermal-spraying surface treatment processes and devices have already been described in the prior art and mention may be made, by way of example, of the documents: EP-A-0,124,432, U.S. Pat. No. 3,744,262, FR-A-2,347,111, EP-A-0,546,359, or else Research Disclosure, January 1997, p. 30, No. 39329.
These various processes or devices are very similar to each other. Schematically, a coolant, generally liquid carbon dioxide, is applied to delivery means which deliver the coolant, generally one or more nozzles within which the liquid carbon dioxide undergoes expansion and creates a two-phase mixture consisting of carbon dioxide gas and dry ice.
In order to obtain a laminar jet, the nozzle is generally in the form of a tube of well-defined geometry: size, shape, etc.
Sometimes, instead of using carbon dioxide in the liquid state, the use of carbon dioxide in the gaseous state only has been described. However, this requires, on the one hand, the use of high pressures, i.e. at least 45 bar, and, on the other hand, the availability of a heating system which allows the storage temperature of the carbon dioxide to be preferably maintained above 30.degree. C. In this case, the expansion of the gaseous carbon dioxide into a mixture of gas and dry ice is carried out through a nozzle, the end of which has at least a flattened shape.
Although the use of cooling by means of a coolant such as CO.sub.2 in a heat-treatment process has a number of advantages, care should nevertheless be taken to ensure that:
the coolant jet emanating from the nozzles comes into intimate contact with the substrate material; PA1 the flow rate and shape of the coolant jet are stable and uniform over time, so as to avoid pulsing, i.e. delivery in spurts, one of the causes of which is the condensation of atmospheric water vapour on the nozzles; PA1 any disturbance of the jet essentially consisting of hot gas and, depending on the case, of molten particles, brought about by the coolant jet is minimal; PA1 the various flow rates are tailored depending on the position of the nozzles delivering the coolant with respect to that of the thermal-spraying guns or nozzles; and PA1 the somewhat flattened or somewhat cylindrical shape of the nozzle delivering the coolant is tailored to the case in question. PA1 spraying means which deliver at least one jet containing at least one hot carrier gas, and PA1 cooling means comprising delivery means which deliver at least one coolant, characterized in that it furthermore comprises shielding means connected to means for supplying at least one gaseous shielding stream, the said shielding means being designed so as to maintain a gaseous shielding atmosphere around at least part of the said delivery means. PA1 the delivery means are one or more delivery nozzles; PA1 the shielding means include a sleeve which surrounds, at least partially, the delivery nozzle or nozzles; PA1 the sleeve is fastened at a proximal end to the delivery means, preferably, upstream of the nozzle; PA1 the sleeve has a free distal end, preferably having a restriction; PA1 the sleeve has a distal end partially closed off by a closing-off means; PA1 the sleeve includes at least one orifice through which the gaseous shielding stream conveyed by the supply means is introduced; PA1 the delivery nozzle includes an end having a circular or oval cross-section or having a flattened cross-section. PA1 at least part of the surface of the material is sprayed from at least one jet containing at least one hot carrier gas; PA1 at least part of the material is cooled by means of at least one delivery nozzle which delivers a coolant; and PA1 at least part of the delivery nozzle is maintained under a gaseous shielding atmosphere by means of at least one shielding gas. PA1 the delivery nozzle is maintained under a gaseous shielding atmosphere by flushing the nozzle with the gaseous shielding stream. PA1 the hot spray jet furthermore includes particles of an at least partially molten material, or softened particles, i.e. in a "pasty" form, and, preferably, of a material selected from the group formed by metals, metal alloys, ceramics, plastics or polymers, silica and metal oxides; PA1 the coolant is selected from nitrogen, carbon dioxide, argon and mixtures thereof; PA1 the flushing is performed by means of at least one dry gas, preferably a gas selected from the group formed by dry air, nitrogen, helium, argon and mixtures thereof. In general, the gases or gas mixtures are also suitable for making it possible to modify the wettability of the molten particles or the cooling with respect to the substrate material; PA1 the gaseous shielding atmosphere, in the sleeve, is at a pressure of greater than 0.9.times.10.sup.5 Pa, preferably greater than or equal to 10.sup.5 Pa, advantageously within the range 1.1.times.10.sup.5 Pa to 3.times.10.sup.5 Pa and more advantageously within the range 1.1.times.10.sup.5 Pa to 2.times.10.sup.5 Pa; and PA1 the flow rate of the gaseous shielding stream depends on the geometry, in particular on the diameter, of the nozzle. Thus, the flow rate of the shielding stream preferably lies within the 5 l/min to 30 l/min range for a nozzle having a diameter of 0.5 mm to 30 mm and within the 8 l/min to 25 l/min range for a nozzle having a diameter of 1 mm to 10 mm.
However, there are still a number of problems arising in this field, problems which hitherto have not been solved.
Among these, the most important is that of the condensation of water vapour, present in the atmospheric air, which occurs on the nozzle or nozzles delivering the liquid coolant and which causes them to ice up.
Such icing of the nozzles is highly harmful since it generally causes formation of a plug which makes the jet of coolant, such as CO.sub.2, unstable and turbulent, thereby resulting, on the one hand, in incorrect and not very effective cooling of the substrate material and/or of the coating layer and, on the other hand, in possible prejudicial disturbance of the heat-shield jet.
Currently, any ice formed by the condensation of water vapour in the atmosphere is removed, very briefly, for example by taking the ice-covered nozzle close to an external heat source, which involves stopping the production line and therefore results in a waste of time and a loss of productivity, and hence in an unacceptable increase in the production costs.